Metal-air cells are well-known and provide a relatively light-weight power supply. Metal-air cells utilize oxygen from ambient air as a reactant in an electrochemical reaction. Metal-air cells include an air permeable electrode as the cathode and a metallic anode surrounded by an aqueous electrolyte and function through the reduction of oxygen from the ambient air which reacts with the metal to generate an electric current. For example, in a zinc-air cell, the anode contains zinc, and during operation, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air permeable cathode.
Early rechargeable metal-air cells included three electrodes, namely, an anode, a unifunctional cathode, and a counter-electrode. The unifunctional cathode was used only during discharge and was incapable of recharging the cells. The counter-electrode was required to recharge the cell. The use of a counter-electrode increased the dead-weight of the cell and reduced the energy density of the cell. To overcome this problem, bifunctional air electrodes were developed for use in metal-air cells. Bifunctional electrodes function in both the discharge mode and the recharge mode of the cell and eliminate the need for the third electrode. However, early bifunctional electrodes did not last long because the recharge reaction deteriorated the discharge system.
U.S. Pat. No. 4,341,848 to Lui et al discloses a bifunctional metal-air electrode comprising carbon particles, a bonding/non-wetting agent, and two types of catalyst, one for oxygen reduction during discharge and one for oxygen evolution during recharge. In that patent, the oxygen reduction catalysts include silver, platinum, platinum-ruthinium, nickel spinel, nickel perovskites, and iron, nickel, or cobalt macrocyclics. The oxygen evolution catalysts include tungsten compounds such as CoWO.sub.4, WC, WS.sub.2, and WC containing fused cobalt. The oxygen reduction catalysts require a relatively high voltage to evolve oxygen. The oxygen evolution catalysts require a lower voltage to evolve oxygen. Thus, during recharging, the oxygen evolution catalysts function at the lower voltage to produce oxygen and recharge the cell and exclude the oxygen reduction catalysts from participating in the recharging reaction. Because the recharging is performed at the lower voltage, the cell deteriorates more slowly and is useful for more cycles than a cell that recharges at higher voltages.
One problem with conventional bifunctional electrodes is that such electrodes may evolve gas at the electrolyte side of the air cathode during discharge and form gas pockets between the air cathode and the electrolyte. In a nonflowing electrolyte system, the gas pockets interrupt the chemical reaction between the electrolyte and the air cathode and cause the cell to prematurely fail. Therefore, there is a need for a bifunctional air electrode that does not prematurely fail due to the production of gas between the electrolyte and the electrode. In addition, it is desired that such an electrode provide sufficient power production on the first discharge cycle and operate for a large number of discharge/recharge cycles.